The PS guns are great - king of the MAC types. They're built like tanks and one of the most recognizable subguns of all time. I don't care what anyone says, the PS MAC-10 is a true classic. All the other MAC types are just copies of it
A forward grip might be a good idea for better control, but if you have a good wooden stock even with the flexible nylon front strap you can have pretty good control. I don't know where you can get a wooden stock anymore, but I friend of mine has one on his PS MAC-10, and its not really so complicated you couldn't make one yourself. Basically get a spare wire stock (or even just the rails from the stock) and use a bolt or screw to attach a wooden stock of your own. You could probably made or modify a wooden stock from a cheap AK stock.
You probably wouldn't need a rate reducer, and unless you could find one designed as a completely drop-in upper for the MAC-10, I wouldn't bother. The rate reducing uppers I know of are all for the SWD M11 guns, and while you could do a TASK-style rate reduction system on a PS MAC-10, that conversion requires you to drill a small hole in the rear of the receiver. Since the PS MACs are becomming collector's items, I don't think I'd advise any permanent modification of the lower receiver like that. An SWD M11 might be a different story, but I wouldn't mess with a PS MAC.
Other than a good solid stock, a suppressor would be top of my list. The suppressor will make it even more controllable and eliminate the need for any kind of k-grip. And besides, while a MAC-10 is a classic, a MAC-10 with a suppressor is just more so.
Stoneycreek has some good uppers, but I'd leave the original upper on the gun as is, and buy one of the stoneycreek uppers with the HK sights and a side-cocker as a spare/extra upper. Try to get a spare bolt so you have two seperate and complete upper units for it. Use one as a real shooter with good sights and maybe a rail for a red dot, and the original upper for the classic, crude, down-and-dirty MAC-10 look and style.
Try to get some spare parts or a complete parts set. Other that that, all I would add is ammo...and lots of it.
BTW - post some pics if you have any!